Signal cables are used to transport signals from a source, such as a detector, sensor or microphone, to a load, such as a signal processing unit or a pre-amplifier. The signal cable must be able to transfer the signal without adding any disturbance or noise to the signal, because the disturbance or noise could change the amplitude, frequency or any other characteristic of the signal. In other words, the signal and its characteristics that are input at one end of the signal cable should be the same as the signal and its characteristics that are output at the opposite end of the signal cable.
One of the sources of noise that may be added to the signal during transport of the signal in the signal cable is tribo-electricity. The tribo-electric effect is an electrical phenomenon in which certain materials can become electrically charged by friction or being rubbed against another material. Movement of a signal cable, for example due to handling of the signal cable or due to movement of the source or load to which the signal cable is connected, leads to mechanical stress of the signal cable, such as bending, compression or stretching, which influences the internal geometry of the signal cable. This may lead to the tribo-electric effect in the signal cable in which electric charge is created which causes an additional, unwanted signal onto the raw signal and can have a significant impact on the signal to noise ratio of the raw signal.
Another source of noise is the change of the value of the coupling capacity between individual wires in the cable, between wires and shielding or between wires and surrounding environment of the cable. The changed value of the capacity can lead to electric charge, thus adding noise to the raw signal. Another source of noise is the change of electrical resistance of wires or shielding induced by cable movement. Also the change of the permeability or susceptibility of the insulator material induced by motion or acceleration can add noise to the raw signal. Furthermore, movement of the cable can change its position with respect to the surrounding environment and in this way change the permeability or susceptibility, again adding noise to the raw signal. Next to noise generated by a movement of the cable, also other sources can affect the signal in the signal cable by introducing additional noise, such as for example electromagnetical interference.
For example when physiological signals of patients are monitored, it is important to get the raw physiological signal at the patient monitor output. Any movement of the signal cables used in this patient monitoring induces a noise signal that is mixed with or added to the raw physiological signal in the signal cable, and affects the signal to noise ratio of the raw physiological signal. Especially in the case of mobile patients, wearing a physiological signal detection unit such as a pulse oximeter for measuring the oxygen saturation of the blood, the signal cable is subjected to movements, like bending, stretching, thereby inducing tribo-electric and other effects in the signal cable that is attached to the signal detection unit and thus adding noise to the raw signal. For the surveillance of these non stationary patients, small and mobile monitoring systems, for example pulse oximeter devices, are operated by small and lightweight batteries which require a low power usage to increase the lifetime of the batteries.
US 2007/0252606 A1 discloses a tri-axial cable used in semiconductor test equipment. The tri-axial cable includes a center signal conductor on which testing signals are carried from test equipment to a shielded probe. The central signal conductor is surrounded by a first dielectric layer, which is surrounded by an electrically conductive coating or dispersion layer. The conductor coating or dispersion layer is sandwiched between the first dielectric layer and an electrically conducting guard layer to reduce tribo-electric effects. In a testing operation, the guard layer is driven at the same potential as the center signal conductor such that the capacitance between the guard layer and the center layer conductor is eliminated. Accordingly, the parasitic capacitance is eliminated. This cable design reduces tribo-electric effects in an active way, thus increasing the power consumption of the cable.